style.css

body{
    background-image: url("assets/land.png");
}

.cat{
    width: 80px;
    height: 100px;
    background-image: url("assets/cat_down.png");
    background-size: 240px 100px;
    background-position: -80px 0;
    position: fixed;
    z-index: -1;
}
.cat.active{
    animation: .2s walk linear infinite;
}

@keyframes walk{
    0%, 32%{
        background-position: 0 0;
    }
    33%, 65%{
        background-position: -80px 0;
    }
    66%, 100%{
        background-position: -160px 0;
    }
}

/* ^ each defines a single frame of the walk keyframe */

.pillow{
    width: 50px;
    height: 40px;
    background-image: url("");
    background-size: 100% 100%;
    position: fixed;
    z-index: -2;
}

.mouse{
    width: 30px;
    height: 23px;
    background-image: url("assets/mouse.png");
    background-size: 100% 100%;
    position: fixed;
    z-index: -3;
}